Modifying the Character Collision Bounds
The sizes of the bounding box types directly rely on the shape of the mesh. In your case the cylinder will align around suzanne's mesh. 

Be aware Cylinder will be symmetrical on single axis. This means the origin of the object should be in the center of the mesh. 

The Cylinder type is homogeneous on X and Y axis too. This means the shape will be a circle rather than an oval. 

Therefore the Cylinder fits most to meshes that forms a nearly a cylindrical shape.

The alternative would be convex hull or triangle mesh (needs more calculation power).

Another alternative is to assign a custom physics mesh (e.g. via parent actuator).